The present invention relates to a novel ultraviolet-curable cellulosic coating composition having excellent adherence to metals, woods, polar plastics, etc. and being capable of forming a film excellent also in the solvent resistance, gloss and impact resistance.
Since cellulose esters are not only derived from cellulose that is one of the recyclable natural resources as a starting raw material, but also capable of forming films excellent in the solvent resistance, gloss, impact resistance, etc., they are applied to paint and ink additives, moldings such as film, and in other wide fields. Moreover, aiming at additional high functionalization of these cellulose esters, many crosslinking reactions that utilize the hydroxyl groups remaining in cellulose ester and being rich in reactivity are known.
In order to achieve good solvent solubility, the degree of ester substitution of cellulose is usually designed as low as around 1, and, as a result, the number of hydroxyl groups in one molecule of cellulose esters is very high. For this reason, in most crosslinking reactions utilizing cellulose esters, the control of crosslinking density is difficult, leading to many problems in actual application.
In order to solve the problems aforementioned, a technology for controlling the number of hydroxyl groups in cellulose acetate to about 1 or 2 at the end of molecule, and further a modifying technology for utilizing these esters are reported in U.S. Pat. No. 3386932, Japanese Unexamined Patent Publication Nos. Sho 60-135423 and Hei 2-48012, R. Amick et al; Polymer, 21, 648 (1980), etc.
With these conventional technologies, however, there arise problems of causing gelation at the stage of modifying these esters through urethane reaction etc., and the like, since it is difficult to accurately control the number and the position of hydroxyl groups in one molecule of cellulose acetate. In addition, these cellulose acetates being near 3-substitution form hardly dissolve into the solvents used commonly, thus making these technologies not to be used widely.
While, in the field of UV-curing resins, a cationic UV-curing resin comprising a combination of epoxy resin, hydroxyl group-containing crosslinking agent and photo initiator is developed widely in recent years from advantages such as less influence of oxygen etc. over conventional radical-polymerizable UV-curing resins. With respect to the application of cellulose esters in this field, there are little actual results in use, and they were at a level of insignificant use as additives. This is because of that, since the common cellulose esters have many hydroxyl groups as described above, if the use level becomes high, the crosslinking density would become too high, resulting in decreased physical properties of film.
As a result of diligent investigations, the inventors have found that cellulose acetate with adjusted number of hydroxyl groups in it has excellent compatibility with particular epoxy compound, hence it forms a homogeneous mixture even without solvent, and further that an excellent film can be formed under UV irradiation by adding photo initiator to this mixture, leading to the invention based on these knowledges.